1. Field of the Invention
The present invention relates generally to a gas turbine engine, and more specifically to a turbine blade with platform cooling.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
In a gas turbine engine, such as a large frame heavy-duty industrial gas turbine (IGT) engine, a hot gas stream generated in a combustor is passed through a turbine to produce mechanical work. The turbine includes one or more rows or stages of stator vanes and rotor blades that react with the hot gas stream in a progressively decreasing temperature. The efficiency of the turbine—and therefore the engine—can be increased by passing a higher temperature gas stream into the turbine. However, the turbine inlet temperature is limited to the material properties of the turbine, especially the first stage vanes and blades, and an amount of cooling capability for these first stage airfoils.
The first stage rotor blade and stator vanes are exposed to the highest gas stream temperatures, with the temperature gradually decreasing as the gas stream passes through the turbine stages. The first and second stage airfoils (blades and vanes) must be cooled by passing cooling air through internal cooling passages and discharging the cooling air through film cooling holes to provide a blanket layer of cooling air to protect the hot metal surface from the hot gas stream.
Prior art turbine rotor blades provide cooling for the platform using several methods. FIGS. 1 and 2 show film cooling holes 13 that open onto the hot surface of the platform 11 on a pressure side of the airfoil 12 supplied with cooling air from the dead rim cavity located below the platform. Film cooling holes supplied from the dead rim cavity requires a higher dead rim cavity pressure than the platform external hot gas pressure. The higher pressure in the dead rim cavity induces a high leakage flow around the blade attachment region and therefore causes engine performance to decrease.
FIGS. 3 and 4 shows another method for cooling blade platforms that uses cooling channels 14 with a long length to diameter ratio to provide convection cooling of the platform hot surface. These long cooling channels are drilled into the platform from the edge side and into the airfoil cooling core. Drilling the convection cooling holes produces unacceptable stress levels and yields a low blade life. This shortened blade life is primarily due to the large mass at the front and back end of the blade attachment which constrains the blade platform edge expansion. The cooling channels are oriented transverse to the primary direction of the stress field so that high stress concentrations associated with the cooling channels are formed at the entrance and exit locations.